Grease composition and rolling bearing

ABSTRACT

A grease composition includes a base oil, a thickener, and polymethyl methacrylate particles. The base oil is a poly-α-olefin. The polymethyl methacrylate particles have an average particle size of 2 μm to 7 μm. A content of the polymethyl methacrylate particles is 10% by mass to 20% by mass with respect to a total amount of the base oil and the thickener.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2019-120098 filed on Jun. 27, 2019, incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The disclosure relates to a grease composition and a rolling bearing in which the grease composition is sealed.

2. Description of Related Art

In recent years, with the growing demand for electric vehicles (EVs) and hybrid vehicles, there is a demand for measures against electrical erosion in motor bearings. There are known anti-electrical erosion techniques that enhance insulation properties between inner and outer rings of a rolling bearing as much as possible to increase withstand voltage and techniques that increase electric flowability between the inner and outer rings of the rolling bearing while frequently repeating small discharges so that electric charge does not accumulate between the inner and outer rings (for example, see paragraph [0003] of Japanese Unexamined Patent Application Publication No. 2012-237334 (JP 2012-237334 A)).

SUMMARY

As described above, although methods for measures against electrical erosion have been proposed, there is still a high demand for improvement in the techniques. Thus, the inventors of the disclosure made studies using a specific composition for the grease to be sealed in the rolling bearing to enhance insulation properties between the inner and outer rings of the rolling bearing and suppress occurrence of electrical erosion in the rolling bearing. Further, when ensuring the insulation properties between the inner and outer rings of the rolling bearing with a grease composition, it is required not only to ensure the insulation properties but also to restrain deterioration in the performance of the rolling bearing.

The inventors have found that a grease composition containing predetermined polymethyl methacrylate particles as an additive ensures insulation properties between the inner and outer rings of the rolling bearing without deteriorating the performance of the rolling bearing, enabling electrical erosion to be suppressed. The inventors completed the disclosure based on this finding.

A grease composition according to a first aspect of the disclosure includes a base oil, a thickener, and polymethyl methacrylate particles. The base oil is a poly-α-olefin. The methyl methacrylate particles have an average particle size of 2 μm to 7 μm. A content of the polymethyl methacrylate particles is 10% by mass to 20% by mass with respect to the total amount of the base oil and the thickener.

The grease composition of the above aspect contains a predetermined amount of the polymethyl methacrylate particles of a specific size in addition to the base oil and the thickener. Thus, the above grease composition has a large volume resistivity and can increase the oil film thickness when used in a rolling bearing. When the above grease composition is used in a rolling bearing, the grease composition forms a thick oil film on friction surfaces (contact surfaces between the inner ring and rolling elements and contact surfaces between the outer ring and the rolling elements) of the rolling bearing. Therefore, the insulation properties between the inner and outer rings can be enhanced, and thus the occurrence of electrical erosion can be suppressed.

In the grease composition of the above aspect, the thickener may be a diurea represented by the following structural formula (1):

R¹—NHCONH—R²—NHCONH—R³  (1)

wherein, in the formula (1), R¹ and R³ are each independently a functional group represented by the following formula (2) and R² is —C₆H₄—CH₂—C₆H₄—;

a bonding position of each —C₆H₄— contained in R² with N is a 4-position when the bonding position of —C₆H₄— with —CH₂— is considered as a 1-position; and in the formula (2), R⁴ is an alkyl group having 1 carbon atom to 12 carbon atoms. The grease composition containing the aromatic diurea represented by the structural formula (1) as a thickener is suitable for increasing the oil film thickness.

In the grease composition of the above aspect, a content of the thickener may be 10% by mass to 30% by mass with respect to the total amount of the base oil and the thickener. The grease composition may further include at least one of a rust inhibitor and an antioxidant. A rolling bearing according to a second aspect of the disclosure is a rolling bearing in which the grease composition of the first aspect is sealed.

According to the grease composition of the above aspect, it is possible to suppress the occurrence of electrical erosion in the rolling bearing in which the grease composition is sealed.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein:

FIG. 1 is a sectional view showing a ball bearing according to an embodiment of the disclosure;

FIG. 2 is a graph for describing a preparation process of a base grease; and

FIG. 3 is a graph showing results of evaluating oil film thicknesses of grease compositions prepared in Examples and Comparative Examples.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the disclosure will be described with reference to the drawings. The rolling bearing according to the embodiment of the disclosure is a ball bearing in which a grease made of the grease composition according to the embodiment of the disclosure is sealed. FIG. 1 is a sectional view showing a ball bearing according to the embodiment of the disclosure. A ball bearing 1 includes an inner ring 2, an outer ring 3 provided radially outside the inner ring 2, a plurality of balls 4 provided between the inner ring 2 and the outer ring 3 and serving as rolling elements, and an annular cage 5 for holding the balls 4. A seal 6 is provided in the ball bearing 1 on a first side and a second side in the axial direction. A grease G made of the grease composition according to the embodiment of the disclosure is sealed in an annular region 7 between the inner ring 2 and the outer ring 3.

The inner ring 2 has, on its outer periphery, an inner raceway surface 21 on which the balls 4 roll. The outer ring 3 has, on its inner periphery, an outer raceway surface 31 on which the balls 4 roll. The balls 4 are interposed between the inner raceway surface 21 and the outer raceway surface 31 and roll on the inner raceway surface 21 and the outer raceway surface 31. The grease G sealed in the region 7 is also provided at contact positions between the balls 4 and the inner raceway surface 21 of the inner ring 2 and at contact positions between the balls 4 and the outer raceway surface 31 of the outer ring 3. The grease G is supplied so as to occupy 20% by volume to 40% by volume with respect to the volume of the space, excluding the balls 4 and the cage 5, surrounded by the inner ring 2, the outer ring 3, and the seals 6. Each of the seals 6 is an annular member including an annular core metal 6 a and an elastic member 6 b fixed to the core metal 6 a. Each of the seals 6 has its radially outer portion fixed to the outer ring 3 and its radially inner portion provided so as to be slidable with respect to the inner ring 2. The seals 6 restrain the supplied grease G from leaking to the outside.

A grease made of the grease composition according to the embodiment of the disclosure described later as the grease G is sealed in the ball bearing 1 configured as described above. Therefore, in the ball bearing 1 in which the grease G is sealed, a thick oil film is formed at the contact positions between the balls 4 and the inner raceway surface 21 of the inner ring 2 and at the contact positions between the balls 4 and the outer raceway surface 31 of the outer ring 3. The occurrence of electrical erosion is thus suppressed.

Next, the grease composition constituting the grease G will be described in detail. The grease composition constituting the grease G is the grease composition according to the embodiment of the disclosure, and contains a base oil, a thickener, and polymethyl methacrylate particles. Since the grease composition contains the polymethyl methacrylate particles, the grease composition has high volume resistivity and excellent insulation properties.

The base oil is a poly-α-olefin (PAO). The poly-α-olefin has no polar group in its molecules and therefore is suitable as a base oil for providing a grease composition having high volume resistivity and excellent insulation properties.

Examples of the poly-α-olefin include those obtained by oligomerizing or polymerizing α-olefin such as 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, or 1-tetradecene and also include hydrogenated poly-α-olefin obtained above. Any one of PAO4 to PAO8 obtained by oligomerizing 1-decene is preferably used as the poly-α-olefin.

The kinematic viscosity of the base oil at 40° C. is preferably 15 mm²/s to 50 mm²/s. This is because the above kinematic viscosity is suitable for reducing torque of the bearing. The kinematic viscosity of the base oil (at 40° C.) is more preferably 25 mm²/s to 35 mm²/s. The above kinematic viscosity of the base oil is a value based on Japan Industrial Standards (JIS) K 2283.

A diurea is preferably used as the thickener. Specifically, a diurea represented by the following structural formula (1) is preferably used.

R¹—NHCONH—R²—NHCONH—R³  (1)

(In the formula (1), R¹ and R³ are each independently a functional group represented by the following formula (2):

(in the formula (2), R⁴ is an alkyl group having 1 to 12 carbon atoms), and R² represents —C₆H₃(CH₃)—, or —C₆H₄—CH₂—C₆H₄—.)

Here, when R² is —C₆H₃(CH₃)—, the phenylene groups are preferably bonded at the 2,4-position or the 2,6-position. When R² is —C₆H₄—CH₂—C₆H₄—, both phenylene groups are preferably bonded at para-position. R² is preferably —C₆H₄—CH₂—C₆H₄—. R⁴ is preferably a methyl group (1 carbon atom (C1)) or a dodecyl group (12 carbon atoms (C12)), and a methyl group is more preferable.

The diurea represented by the above structural formula (1) is an aromatic diurea in which R¹ and R³ are aromatic functional groups. The grease composition using such an aromatic diurea is easily interposed between the outer ring and the rolling elements and between the inner ring and the rolling elements, and is suitable for securing a sufficient oil film thickness at these portions. With the grease G having high volume resistivity provided in these portions so as to have a sufficient oil film thickness, the rolling bearing in which the grease G is sealed has excellent insulation properties between the inner and outer rings.

The diurea represented by the above structural formula (1) is a reaction product of an aromatic amine and a diisocyanate compound. The aromatic amine is an aromatic amine represented by the following formula (3):

(in the formula (3), R⁵ is an alkyl group having 1 carbon atom to 12 carbon atoms.) Preferable examples of the aromatic amine represented by the above formula (3) include 4-amino-1-methylbenzene (p-toluidine), 2-amino-1-methylbenzene (o-toluidine), 4-amino-1-dodecylbenzene, and 2-amino-1-dodecylbenzene. These aromatic amines can be used alone or in combination of two or more thereof.

Specific examples of the diisocyanate compound include 2,4-toluene diisocyanate (2,4-TDI), 2,6-toluene diisocyanate (2,6-TDI), a mixture of 2,4-TDI and 2,6-TDI, and 4,4′-diphenylmethane diisocyanate (MDI).

To obtain the aromatic diurea represented by the structural formula (1), the reaction of the aromatic amine and the diisocyanate compound can be performed under various conditions. The reaction is preferably performed in the base oil to obtain a diurea compound with highly-uniform dispersibility as a thickener. The reaction between the aromatic amine and the diisocyanate compound may be performed by adding a base oil in which the diisocyanate compound is dissolved to a base oil in which the aromatic amine is dissolved, or by adding a base oil in which the aromatic amine is dissolved to a base oil in which the diisocyanate compound is dissolved.

The temperature and duration for the reaction between the aromatic amine and the diisocyanate compound are not particularly limited, and conditions similar to those usually used in this type of reaction can be used. The reaction temperature is preferably 150° C. to 170° C. in view of the solubility and volatility of the aromatic amine and the diisocyanate compound. The reaction time is preferably 0.5 hours to 2.0 hours in view of completing the reaction between the aromatic amine and the diisocyanate compound, and improving production efficiency of the grease by reduced production time.

The content of the thickener is preferably 10% by mass to 30% by mass with respect to the total amount of the base oil and the thickener. When the content of the thickener is less than 10% by mass, the ability of the grease to retain the base oil decreases, and there is a high possibility that a large amount of the base oil separates from the grease during rotation of the rolling bearing. In contrast, when the content of the thickener exceeds 30% by mass, an increase in the stirring resistance generated by shearing of the grease due to relative movement of the inner ring, the outer ring, the balls, and the cage caused by the rotation of the rolling bearing may increase the torque of the rolling bearing. Further, heat generated in the grease due to the stirring resistance generated by the shearing of the grease may accelerate oxidization of the grease or deterioration of the grease due to evaporation of the base oil and oil separation. A more preferable content of the thickener is 20% by mass to 30% by mass with respect to the total amount of the base oil and the thickener.

The grease composition contains a predetermined amount of polymethyl methacrylate particles of a specific size. By containing the above polymethyl methacrylate particles, the grease composition can increase the oil film thickness. Further, the polymethyl methacrylate particles are particles softer than inorganic particles such as carbon black, and when the grease composition is sealed in the rolling bearing, the polymethyl methacrylate particles are unlikely to cause wear in components of the rolling bearing or to reduce the acoustic performance. The grease composition is suitable for suppressing the occurrence of electrical erosion while satisfying the basic performances required for application to a rolling bearing.

The polymethyl methacrylate particles have an average particle size of 2 μm to 7 μm. Setting the average particle size of the polymethyl methacrylate particles in the above range is suitable for increasing the oil film thickness of the grease composition. When the average particle size of the polymethyl methacrylate particles is less than 2 μm, the oil film thickness of the grease composition is unlikely to increase. Specifically when the rolling bearing rotates at low speed, there are cases in which a sufficient oil film thickness cannot be secured. In contrast, when the average particle size exceeds 7 μm, it becomes difficult for the grease composition sealed in the rolling bearing to intervene on the friction surfaces of the rolling bearing. The average particle size of the polymethyl methacrylate particles is preferably 6 μm or smaller. The average particle size of the polymethyl methacrylate particles is an arithmetic average value of the particle diameters of the polymethyl methacrylate particles. The average particle size is obtained by averaging the diameters of spheres obtained by replacing each of the polymethyl methacrylate particles with a sphere having an equivalent volume using the Mastersizer 3000 laser diffraction particle size analyzer manufactured by Malvern Panalytical (Spectrics PLC).

The content of the polymethyl methacrylate particles is 10% by mass to 20% by mass with respect to the total amount of the base oil and the thickener. When the content of the polymethyl methacrylate particles is less than 10% by mass, the effect of increasing the oil film thickness and increasing the insulation properties is poor. In contrast, when the content of the polymethyl methacrylate particles exceeds 20% by mass, the grease may be hardened to deteriorate lubricity, or the stirring resistance may be increased to increase the rotational torque. Furthermore, it may be difficult to maintain properties of a grease suitable for being sealed in a rolling bearing. The content of the polymethyl methacrylate particles is preferably 10% by mass to 15% by mass with respect to the total amount of the base oil and the thickener.

The grease composition may further contain a rust inhibitor or an antioxidant. By adding the rust inhibitor, generation of rust in the rolling bearing in which the grease composition is sealed can be suppressed. By adding the antioxidant, the lubrication life of the grease composition can be improved. The grease composition may further contain, as other additives, an extreme pressure additive, an oil agent, an antiwear agent, a dye, a hue stabilizer, a thickening agent, a structural stabilizer, a metal deactivator, a viscosity index improver, and the like.

Next, a method for producing the grease composition will be described. As the procedure of the production of the grease composition, for example, first, a base grease composed of the base oil and the thickener is prepared, and then the polymethyl methacrylate particles is added to the obtained base grease together with any additive to be contained as necessary. The obtained mixture is then stirred with a rotation/revolution mixer or the like to mix the components, thereby the grease composition is produced.

According to the present embodiment, as the grease composition constituting the grease G sealed in the ball bearing 1, a grease composition containing a predetermined amount of the above-mentioned polymethyl methacrylate particles in addition to the above-mentioned base oil and thickener is used. By using such a grease composition, in the ball bearing 1 in which the grease G is sealed, the insulation properties between the inner and outer rings can be enhanced, and the occurrence of electrical erosion can be suppressed.

The disclosure is not limited to the above embodiment, and can be implemented according to other embodiments. The rolling bearing according to the embodiment of the disclosure is not limited to the ball bearing in which the grease composed of the grease composition according to the embodiment of the disclosure is sealed. The rolling bearing may be other rolling bearings in which rolling elements other than balls are used, such as a roller bearing, as long as a grease composed of the grease composition according to the embodiment of the disclosure is used.

Next, the disclosure will be described in more detail with reference to Examples. Note that the disclosure is not limited to the Examples. Here, a plurality of grease compositions were prepared, and the characteristics of each grease composition were evaluated. Table 1 shows the composition and the evaluation result of each grease composition.

Preparation of Base Grease

As a base grease, the grease composition containing the base oil and the thickener was prepared with the following process. FIG. 2 is a graph for describing a preparation process of the base grease.

(1) PAO6 (product name: Durasyn 166 polyalphaolefin produced by Ineos Oligomers, kinematic viscosity at 40° C.: 29 mm²/s to 33 mm²/s), which is one type of poly-α-olefin, is used as the base oil. The base oil is heated so that its temperature becomes 100° C. (2) Predetermined amounts of the base oil, p-toluidine, and 4,4′-diphenylmethane diisocyanate (MDI) are weighed out. (3) Half the amount of the base oil (100° C.) and the MDI are poured into a stainless steel container A and the mixture is stirred at 100° C. for 30 minutes. (4) The other half of the base oil (100° C.) and the p-toluidine are poured into a different stainless steel container B and the mixture is stirred at 100° C. for 30 minutes. The process (3) and (4) described above are referred to as a first process. (5) An amine solution in the stainless steel container B is poured into the stainless steel container A, and the amine solution is gradually added to the isocyanate solution. At this time, the liquid temperature rises by about 20° C. due to reaction heat. (6) After checking that the entire amount of the amine solution in the stainless steel container B has been poured into the stainless steel container A, the mixture in the stainless steel container A is heated so that the temperature of the mixture becomes 170° C. (7) The mixture is stirred while being heated, and the temperature of the mixture is maintained at 170° C. for 30 minutes. This process (7) is referred to as a second process. (8) Heating of the mixture is stopped and the mixture is cooled naturally while being stirred. The mixture is cooled until its temperature becomes 100° C. (9) After checking that the temperature of the mixture has decreased to 100° C. or lower, the stirring is stopped and the mixture is cooled naturally to room temperature. (10) A homogenization treatment is performed with a three-roll mill. At this time, the processing conditions are as follows. Clearance between rolls: 50 μm Roll pressure: 1 MPa Rotation speed: 200 r/min Processing temperature: room temperature (RT) With the above process (1) to (10), the base grease was prepared. The base grease was subjected to evaluation described later as the grease composition of Comparative Example 1.

Grease compositions of Examples 1 and 2 and Comparative Examples 2 and 3 were prepared using the above base grease. At this time, the following reagents were used.

-   -   Polymethyl methacrylate particles (A): MB-4 (produced by Sekisui         Plastics Co., Ltd., average particle size=4.9 μm)     -   Polymethyl methacrylate particles (B): MBP-8 (produced by         Sekisui Plastics Co., Ltd., average particle size=8.0 μm)

Example 1

A grease composition was prepared by mixing 100 parts by mass of the above base grease with 10 parts by mass of polymethyl methacrylate particles (A). Here, the polymethyl methacrylate particles (A) were mixed with the base grease using a rotation/revolution mixer at a rotation speed of 2000 rpm for 3 minutes.

Example 2

A grease composition was prepared in the same manner as in Example 1 except that the content of the polymethyl methacrylate particles (A) was changed to 20 parts by mass.

Comparative Example 2

A grease composition was prepared in the same manner as in Example 1 except that the content of the polymethyl methacrylate particles (A) was changed to 5 parts by mass.

Comparative Example 3

A grease composition was prepared in the same manner as in Example 1 except that the polymethyl methacrylate particles (B) were added instead of the polymethyl methacrylate particles (A).

Evaluation of Grease Composition

The volume resistivities and the oil film thicknesses of the grease compositions prepared in Examples 1 and 2 and Comparative Examples 1 to 3 were evaluated by the following methods. The results are shown in Table 1 and FIG. 3.

TABLE 1 Example Example Comparative Comparative Comparative Item Unit 1 2 Example 1 Example 2 Example 3 Thickener Aromatic diurea wt % 28 Base oil Poly-α-olefin wt % 72 Additive Polymethyl methacrylate wt % 10 20 0 5 0 particles (A) (Average particle size: 4.9 μm) Polymethyl methacrylate wt % 0 0 0 0 10 particles (B) (Average particle size: 8.0 μm) Evaluation Volume resistivity Ω · cm 6 × 10¹⁵ 6 × 10¹⁵ 4.0 × 10¹⁵ 6 × 10¹⁵ 5.0 × 10¹⁵ or larger or larger or larger Oil film thickness nm 1671 1179 934 289 501 *The unit “wt %” of the substances in Table 1 indicates the “wt %” with respect to the total amount of the base oil and the

Measurement of Volume Resistivity

The volume resistivities of the grease compositions prepared in Examples and Comparative Examples were measured by the following method. “Liquid resistance sample box 12707 manufactured by ADCMT” was used as an electrode and “digital ultra-high resistance/micro current meter R8340A manufactured by ADCMT” was used as a measuring device. An amount of 0.8 ml of a grease composition serving as a sample was poured into the liquid resistance sample box. The volume resistivity (Ω·cm) of the grease composition was then measured. The measurement conditions are as shown in Table 2.

TABLE 2 Item Conditions Measuring device Digital ultra-high resistance/micro current meter 8340A manufactured by ADCMT Electrode Liquid resistance sample box 12707 manufactured by ADCMT Amount of grease 0.8 ml Thickness of test sample 1000 μm Measuring voltage 10 V Measuring gain ×1000 Use of guard electrode Used

It is considered that the higher the volume resistivity is, the better insulation properties the grease composition has, and the more effect the grease composition has in suppressing electrical erosion when the grease is sealed in a rolling bearing. The maximum value of the volume resistivity that can be measured when the grease compositions are evaluated under the above conditions is 6×10¹⁵ Ω·cm. Therefore, the grease composition with a volume resistivity evaluation result of “6×10¹⁵ or larger” has a volume resistivity exceeding the maximum value that can be measured under the above conditions.

Oil Film Thickness

The oil film thicknesses of the grease compositions prepared in Examples and Comparative Examples were measured using an elastohydrodynamic lubrication (EHL) ultra thin film thickness measurement system (EHD2 produced by PCS Instruments) based on the conditions in Table 3 below. The results are shown in Table 1 and FIG. 3.

TABLE 3 Item Conditions Measuring device EHL ultra thin film thickness measurement system (EHD2 produced by PCS Instruments) Rotating speed 2 mm/s Load 20N Contact pressure 0.51 GPa Temperature 120° C. Disk type Glass rotation plate (SL952) Steel sphere type ¾-inch diameter steel sphere Measured item Thickness at the center of oil film

As shown by a result of Examples and Comparative Examples, the grease composition according to the embodiment of the disclosure has a high volume resistivity and a large oil film thickness. The grease composition according to the embodiment of the disclosure thus has excellent insulation properties, and can suppress the occurrence of electrical erosion in the rolling bearing in which the grease composition is sealed. 

What is claimed is:
 1. A grease composition comprising: a base oil; a thickener; and polymethyl methacrylate particles, wherein the base oil is a poly-α-olefin, the polymethyl methacrylate particles have an average particle size of 2 μm to 7 μm, and a content of the polymethyl methacrylate particles is 10% by mass to 20% by mass with respect to a total amount of the base oil and the thickener.
 2. The grease composition according to claim 1, wherein the thickener is a diurea represented by the following structural formula (1): R¹—NHCONH—R²—NHCONH—R³  (1) wherein, in the formula (1), R¹ and R³ are each independently a functional group represented by the following formula (2) and R² is —C₆H₄—CH₂—C₆H₄—;

a bonding position of each —C₆H₄— contained in R² with N is a 4-position when the bonding position of —C₆H₄— with —CH₂— is considered as a 1-position; and in the formula (2), R⁴ is an alkyl group having 1 to 12 carbon atoms.
 3. The grease composition according to claim 1, wherein the content of the thickener is 10% by mass to 30% by mass with respect to the total amount of the base oil and the thickener.
 4. The grease composition according to claim 1, further comprising at least one of a rust inhibitor and an antioxidant.
 5. A rolling bearing in which the grease composition according to claim 1 is sealed. 